Energy coverage of muscle contraction


Metabolism is a sum of events which are carried out in the human body to create energy and other substances necessary for its activities. In our organism there are catabolic and anabolic processes.

Catabolism is a process during which organic matter is broken down and the energy is simultaneously released. It is characterized by missing reserves of glycogen and mobilisation of non-saccharide sources of energy – fats and proteins. Catabolism takes place during increased movement activity and is necessary to sustain life functions.

Anabolism, on the other hand, is a energy-consuming process during which substances are created. The substrate supply exceeds the immediate need. The organism creates energy reserves, tissues are created and renewed. Anabolic processes are prevalent in situations of reduced physical activity.

The basic nutrients (carbohydrates, lipids, proteins) are present in food we eat. Those are transformed and absorbed through the digestive system. Carbohydrates break down into individual carbohydrates (monosaccharides) where the glucose ranks among the most important ones. Lipids break down into free fatty acids and glycerol. Proteins break down into amino acids. These simple agents can then become involved in more complicated processes.

Carbohydrates are used in both anaerobic and aerobic activities. ATP resynthesizes from glycogen (muscle glycogen, liver glycogen) which transforms into glucose. Supplies of glycogen in the human body are restricted. Lipids are used in endurance-based movement activity of low intensity. While the use of proteins in the ATP resynthesis is very limited, free fatty acids are used to a large extent. Glucose is generated through gluconeogenesis.

Muscle metabolism

Muscles need energy to produce contractions (Fig. 6). The energy is derived from adenosine triphosphate (ATP) present in muscles. Muscles tend to contain only limited quantities of ATP. When depleted, ATP needs to be resynthesized from other sources, namely creatine phosphate (CP) and muscle glycogen. Other supplies of glycogen are stored in the liver and the human body is also able to resynthesize ATP from lipids, i.e. free fatty acids. Different modes of energy coverage are used depending on intensity and duration of the workload put on the organism.

Figure 6 Energy for muscles

The ATP-CP system

The above mentioned ATP and CP are the energy sources of muscle contraction (Fig. 7, 8, 9). The production of energy used in muscle contraction takes place through the anaerobic way (without oxygen).

Figure 7 ATP molecule

Figure 8 ATPase (ATP breakdown and energy production for muscle contraction)

Figure 9 ATP resynthesis from CP

Anaerobic glycolysis

It is a chemical process during which ATP gets renewed from glycogen, i.e. glucose in an anaerobic way (without access to oxygen). In these processes lactate, i.e. salt of the lactic acid is generated in muscles. This energy system produces 2 molecules of ATP. Glycolysis - transformation of glucose into 2 molecules of the pyruvate generating the net yield from ATP molecules and 2 NADH molecules (anaerobic breakdown of glucose into pyruvate and lactate) – see. Fig. 10.

Oxydative system

This is a chemical process during which the ATP resynthesis takes place through an aerobic way (with access to oxygen). Both glycogen or glucose and free fatty acids act here as sources of energy.

Aerobic glycolysis takes place in the cytoplasm of the cell where 34 ATP molecules are generated from the glycogen, i.e. glucose with oxygen present (Fig. 10).

Figure 10 Anaerobic and aerobic glycolysis

Free fatty acids present in mitochondria of muscle fibres transformed into acetyl CoA are used in the ATP resynthesis. Acetyl CoA enters the Krebs cycle and thus ATP molecules are generated.

Individual energy systems get involved according to the intensity of a movement activity carried out. If the performance is conducted at the maximum level, there is a gradual involvement of all the systems (Fig. 11, 12).

Figure 11 Energy coverage under maximum workload

Figure 12 Energy coverage under maximum workload

Types of muscle fibres

Human muscle fibres have distinct qualities. Although nowadays almost 30 types of muscle fibres are known to be present in the human body, we tend to work only with the following three types:

Slow red muscle fibre I (SO - slow oxidative fibres)

The slow red muscle fibre is typified by a high aerobic capacity and resistance to fatigue. As their anaerobic capacity is slow, they are not able to show great muscle strength. Muscle contraction tends to be slow – 110 ms/muscle contraction. One motoric unit contains about 10-180 muscle fibres.

Fast red muscle fibre IIa (FOG – fast oxidative glycolytic fibres)

The fast red muscle fibre shares some of qualities with a slow fibre or a fibre of IIx type. This fibre is typified by medium aerobic capacity and resistance to fatigue. It also shows high anaerobic capacity and is able to display great muscle strength. The speed of contraction is 50 ms/muscle contraction. One motoric unit contains about 300-800 fibres.

Fast white fibre IIx (FG – fast glycolytic fibre)

Unlike the previously mentioned types the fast white fibre is characterized by low aerobic capacity and tendency to fast fatigue. On the other hand, it has the greatest anaerobic capacity and is able to display considerable muscle strength. The speed of contraction is 50 ms/muscle contraction. One motor unit contains about 300-800 fibres.

The volume of this type muscle fibres is genetically given (up to 90 %) (Jančík et al., 2007) and varies in individual persons. In the average population the ratio of slow to fast fibres is 1:1. The following Figure (Fig. 13) shows the ratio of slow to fast fibres in athletes engaged in different disciplines.

Figure 13 Ratio of fast (type FG and FOG) to slow (type SO) fibres in different type athletes

In muscle contraction individual types of muscle fibres get activated in accordance with the intensity of muscle movement. During low intensity exercise slow fibres are primarily recruited. However, with increasing intensity of exercise fast fibres get activated. It is important to note here that the fibre ratio differs in different muscles of the human body. For example, postural muscles tend to contain more slow fibres.